A. Field of the Invention
This invention relates to a method of and apparatus for mitigation of multipath signals from Spread Spectrum Ranging System Code measurements and from Carrier Phase measurements. More specifically this invention relates to a method of and an apparatus for mitigation multipath signals utilizing multiple correlators, for both Ranging System Code and Carrier Phase.
B. Problems in the Art
Multipath is a dominant error source in high accuracy Global Navigation Satellite System (GNSS) applications. Multipath errors or xe2x80x9cmultipath fadingxe2x80x9d in C/A code pseudo-random noise (PRN) tracking with conventional early minus late correlators is often several meters in magnitude. A tracking error is caused by multipath distortion. Errors caused by multipath distortion can be reduced by narrowing the delay spacing between the early and late correlators to less than one chip in the Delay Lock Loop (DLL) detector. (A xe2x80x9cchipxe2x80x9d is the time during which the code remains a plus or minus one.) However, this technique does not eliminate multipath errors.
Throughout the previous decade, varieties of receiver architectures were introduced to mitigate multipath for C/A-code GPS or GLONASS. The so-called Narrow Correlator receiver, discussed above, was introduced in 1991 and its multipath-mitigation properties were documented subsequently. More recently the Multipath Estimating Delay-Lock Loop (MEDLL), Multipath Elimination Technology, Strobe and Edge Correlators, Early1-Early2 Tracker, Multipath Mitigator Types A and B, and Enhanced Strobe Correlator have been showcased. However, it is believed that the performance quoted for the Enhanced Strobe Correlator and the Multipath Mitigator Type B can only be achieved through gating of the incoming signal or a so-called superresolution technique.
Therefore, the primary feature of this present invention is to obtain significant multipath mitigation without the use of gating or utilizing a superresolution technique.
Other features of this present invention include:
measuring a phase angle of a spread spectrum pseudo-random noise (PRN) encoded signal;
mitigation of multipath errors while measuring a phase angle of a PRN encoded signal;
measuring the relative time delay of a spread spectrum signal PRN code signal;
mitigation of multipath errors while the measuring the delay of a spread spectrum PRN code signal;
mitigation of multipath errors while measuring a phase angle of a PRN encoded signal through existing receivers; and
mitigation of multipath errors while measuring time delay of a spread spectrum PRN code signal code through existing receivers; and
complementary filtering for a combination of multipath mitigation and desirable noise characteristics.
A method of measuring phase angle of a pseudo-random noise (PRN) code spread spectrum signal by receiving a PRN signal, generating at least three correlator signals with quadrature phase components, taking linear combinations of the correlator signals to synthesize a high resolution correlator output for each quadrature phase component, and computing a phase tracking error signal from said quadrature phase high resolution correlator outputs.
Also a method of decoding a spread spectrum signal encoded on a PRN binary code, by receiving a PRN signal, generating at least four correlator signals corresponding to the received PRN code, mixing said PRN signals with received signal to obtain at least four measurements of the PRN code auto-correlation function, utilizing linear combinations of the correlation function to synthesize an error signal having a magnitude as a function of the relative timing difference between the incoming PRN code signal and the locally generated PRN reference signals that (a) is zero for a zero relative timing difference and for a majority of a range of the relative time difference between plus and minus on chip, and (b) increases as the relative timing difference changes from zero to another value within a central portion of said range, when the error signal has a non-zero magnitude, adjusting the relative timing difference by adjusting the phase of the locally generated PRN signals in a manner to drive said error signal to zero; and outputting the high resolution correlator (HRC) decoded signal.